The present invention relates to a novel selenium-containing compound. The invention also relates to a method of producing the selenium-containing compound, use of the selenium-containing compound as an antioxidant, and an analysis method that uses the selenium-containing compound as a standard substance.
Selenium is an essential trace element for humans. Selenium forms enzymes and proteins in vivo, and plays an important role in antioxidant reactions (Non-patent Documents 1 and 2). Selenium is abundant in algae, fish, shellfish, meat, and egg yolk (Non-patent Document 3). Selenium is also abundant in the meat of various fish species generally consumed by the Japanese (Non-patent Documents 1 to 4).
It is known that the dark muscle of tuna and whale meat contain selenium in high concentration (Non-patent Documents 5 to 7). The selenium content in each internal organ of Thunnus orientalis (Pacific bluefin tuna) has been reported (e.g., blood (15.2 ppm), kidney (8.3 ppm), spleen (7.6 ppm), superficial dark muscle (6.1 ppm), true dark muscle (5.9 ppm), heart superficial dark muscle (4.4 ppm), liver superficial dark muscle (4.1 ppm), gills (2.6 ppm), brain (1.4 ppm), and ordinary muscle (0.57 ppm)). Since the selenium content exceeds 4 ppm in some internal organs, these internal organs may be used as an organic selenium source (Non-patent Document 8). However, since the biochemical properties of selenium in each internal organ are unclear, and it is difficult to obtain a high-purity extract, these internal organs have not been effectively utilized as an organic selenium source.
Since selenium is normally present in food as a selenocysteine residue (i.e., a constituent amino acid of a protein), it is considered that selenium is digested and absorbed when absorption of the protein occurs. Therefore, the absorption of selenium in food from a digestive tract is estimated to be 50% or more (Non-patent Document 1). About 250 μg/kg of selenium is present in the human body, and in vivo selenium homeostasis is maintained through excretion into urine (Non-patent Document 1).
The selenocysteine residue of an enzyme/protein is positioned at a selenol group (i.e., active center) of a selenoprotein. Selenoproteins (e.g., glutathione peroxidase and thioredoxin reductase that decompose and remove active oxygen and hydroperoxides, 5′-iodothyronine deiodinase that is involved in production of thyroid hormones, and selenoprotein P that is present in plasma) play an important role in an in vivo antioxidant effect (Non-patent Document 1). It has been considered based on the above findings that organic selenium included in fish meat at a high concentration is a selenoprotein (e.g., glutathione peroxidase) or a peptide/amino acid thereof (Patent Document 1).
When selenium deficiency has occurred, cellular disorder occurs due to peroxides (Non-patent Document 1). The onset of cardiomyopathy (Keshan disease) (i.e., selenium deficiency disease) observed in the selenium-deficient northeast part of China is prevented by administration of selenious acid (Non-patent Document 1). Kaschin-Beck disease (Beck's disease) that manifests in adolescence and is observed in selenium-deficient northern China and Siberia is also caused by selenium deficiency (Non-patent Document 1).
It has been reported that selenium deficiency has a correlation with coronary artery diseases (i.e., angina pectoris and heart infarction) based on an epidemiological study that compares the incidence of heart diseases with the selenium level in blood. It has been reported that the rate of deaths from cardiovascular diseases due to selenium deficiency is high in eastern Finland, and a group with a serum selenium level of 45 μg/l or less has a high incidence of heart diseases (Non-patent Document 1).
It is known that muscle pains, skin dryness, liver necrosis, and the like are caused by selenium deficiency (Non-patent Document 1). It has been reported that selenium deficiency increases the risk of cancer such as lung cancer, large bowel cancer, prostate cancer, rectal cancer, breast cancer, and leukocythemia (Non-patent Documents 1 and 10). It has been reported that the cellular immune response to cancer cells is enhanced by administering 200 μg/day of sodium selenite during surgical therapy or radiotherapy of cancer (Non-patent Document 8). It has also been reported that selenium acts on the cancer cell signaling system to suppress cell growth, and induces apoptosis (Non-patent Document 1). Non-patent Document 12 points out that 100 to 200 μg/day of selenium suppresses DNA mutation and oxidative damage due to carcinogens to suppress the progress of cancer, but an intake of more than 400 μg/day of selenium may be harmful. It has thus been considered that selenium is effective for suppressing or treating cancer, or preventing recurrence of cancer.
On the other hand, excessive intake of selenium is toxic, and may cause nail deformation, unhairing, gastrointestinal injury, neuropathy, heart infarction, acute respiratory distress, renal insufficiency, and the like (Non-patent Document 1).
As the dietary reference intakes of selenium, the estimated average requirement is set to 25 (20) μg, the recommended dietary allowance is set to 30 (25) μg, and the upper limit is set to 450 (350) μg (the values are for adult men (the value in parentheses is the estimated average requirement for adult women)) (Ministry of Health, Labour and Welfare, Nov. 22, 2004, Non-patent Document 13). Note that the estimated average requirement and the recommended dietary allowance for a 30 to 49-year-old male is 30 μg and 35 μg, respectively (Non-patent Document 13). In Japan, the selenium upper limit is set to 100 to 450 μg Se/day taking account of the value (800 μg Se/day) obtained by the study on Enshi, Hubei Province in China using unhairing and nail brittleness/loss as indices (Non-patent Documents 13 and 16).
In recent years, an organic selenium-containing supplement that includes inorganic selenium and selenomethionine as active ingredients has been used to treat or prevent diseases due to selenium deficiency. Selenium-containing yeast that is obtained by culturing yeast in a culture medium including inorganic selenium, and includes selenomethionine at a high concentration has been used as an organic selenium-containing compound supply source (Non-patent Document 1). Such selenium-containing yeast has been used to increase the selenium content in food, a cosmetic preparation, and feed.
Organic selenium can be extracted from a raw material (e.g., meat or internal organs of fish) having a selenium content of more than 0.5 ppm in order to supply selenium that can be efficiently used for humans, livestock, fish, and shellfish. A selenium-containing material having a selenium content of more than 5 ppm can be provided by freeze-drying these tissues or concentrating an extract thereof. A protein concentrate and a hydrochloric acid hydrolyzate having a total selenium content of about 18 to 103.5 ppm has been obtained from dark muscle (Patent Document 1). However, since the muscles and the internal organs of large fish and carnivorous fish such as tuna contain methylmercury at a relatively high concentration of more than 0.5 ppm, an unpurified dry powder, freeze-dried product, concentrate, protease hydrolyzate, and the like thereof contain selenium at a relatively high concentration, but also contain methylmercury at a high concentration. Moreover, the internal organs of fish and shellfish have a cadmium content of 1 ppm or more. Specifically, since the dark muscles and the internal organs of fish and shellfish may contain methylmercury and cadmium (toxic heavy metals), the dark muscles and the internal organs of fish and shellfish cannot be suitably used for drugs, food, feed, and the like. Therefore, it is necessary to remove toxic heavy metals by purification from an organic selenium-containing compound derived from a fish/shellfish tissue concentrate/extract.
Non-patent Document 14 discloses a method that analyzes selenium in a living body and food by performing thermal wet-digestion using a mixture of nitric acid and perchloric acid, reacting the resulting product with 2,3-diaminonaphthalene (DAN), and utilizing the fluorescence of 4,5-benzopiaselenol (Se-DAN) produced by a complex-forming reaction with Se(IV). Non-patent Document 15 discloses a method that identifies the molecular species of inorganic and organic selenium in the environment using an ICP-MS that is connected to an HPLC online. However, a method that analyzes organic selenium-containing compounds including proteins and amino acids in a living body and food at the same time has not been proposed.
Selenium has been industrially used as a photoreceptor/semiconductor material, a red to orange pigment for glass, ceramics, and plastics, a decolorizer and an anti-foaming agent used for glass production, a metallurgical additive, and the like (Non-patent Document 16).